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Helicopter rotor blades must be balanced to reduce vibration that can place stress on the helicopter’s airframe. At FRCE, this balancing process has traditionally been performed using the facility’s Helicopter Blade Balance Facility, commonly known as a whirl tower, to simulate the blade’s performance in flight.
“Normally, when you balance a blade, you’re using a physical master blade,” said Joshua Peedin, senior rotor systems engineer for the H-53 Fleet Support Team (FST). “You check it against a static balancer, then you run it on the tower to take into account the dynamic loads. The goal is to make the blade weigh and fly similar to the master blade.”
However, the whirl tower at FRCE doesn’t have the size or the horsepower to accommodate the length and weight of a CH-53K blade; that’s where the universal static balance fixture, or USBF, comes in.
“The USBF fixture creates a virtual master where the known parameters, dimensions and characteristics of the master blade are entered into the software,” said Peedin. “When you weigh a main rotor blade, it compares the data against the virtual parameters to tell you how to adjust the blade and what weights to add or subtract.”
The artisan then adds weights to the tip end of the blade, in positions that affect the lengthwise weight of a blade or how it will pitch forward and back. This process must be precise; the weight difference caused by sanding or applying a repair can spell the difference between balancing a blade or sending it back to the manufacturer for stripping and rebuilding.
The USBF is currently used to balance H-53E main rotor and tail blades, as well as blades for the V-22 Osprey and H-1 Viper and Venom aircraft. The system has performed so well that the maintenance plan for the new CH-53K model calls for its rotor blades to be tested on the USBF, according to FRCE engineers.
The decision whether to adopt the USBF system required extensive testing in order to gather and analyze data, said Peedin.
“We used the data we procured while testing H-53E blades to validate the process,” he said. “As we stepped through and finally tested it on aircraft, everyone became comfortable enough with the results to remove the whirl tower requirement from the CH-53K maintenance plan in favor of the USBF.”
The USBF system’s reliability and comparatively low purchase and maintenance costs are expected to help FRCE meet the fleet’s needs for economical maintenance strategies for the H-53K program, according to Daniel Ventry, CH-53K lead system engineer at the H-53 FST.
“The addition of this system is one of the initiatives that the program is investigating to reduce the total life cycle cost of the new acquisition program,” Ventry said. “As we develop the strategy for the life cycle, one of our tenets of the program strategy is to ensure that the long term sustainment of the program is as cost effective as possible.”
FRCE engineers and artisans say the static fixture is more reliable than the whirl tower, which can be affected by wind, rain or bright sunlight. In addition, crane maintenance can also be a factor in whether the whirl tower can be used on a given day. Conversely, the USBF is housed in a climate-controlled building with four overhead crane systems available for use.
“Just the other day, I had blades up and I got a call from the [air traffic control] tower saying there was a gust of wind coming at 30 knots,” said Robert Call, work leader for the dynamic components shop. “I had to run out there and take them down, or the wind could snap a blade. I can run blades on the new fixture no matter what the weather.”
Unlike the whirl tower, FRCE has a backup USBF fixture available for use in case the primary system requires maintenance or repair. The H-53 Program Office procured two USBF fixtures to mitigate any potential downtime that could result from an inoperable system, said Zackary Barnard, rotor blade shop lead engineer at FRCE. In addition, with a price tag of $170,000 per unit, engineers say the USBF system is more economical than the whirl tower in the long run.
“The cost to run the tower, as far as power draw and maintenance, is around $500,000 a year,” Barnard explained. “The USBF doesn’t have those costs.”
The systems are also portable, which makes them accessible to deployed units. Engineers say this ability to balance blades will allow fleet maintainers to perform more blade repairs on site.
“There are some repairs the fleet couldn’t perform because the blade had to be balanced afterward,” Peedin said. “Now they’ll have the capability to balance the blade rather than sending it back to FRCE for balancing, which means they can do more repairs themselves.”
FRCE engineers are hoping the new system will give them forecasting ability to determine the outcome of repairs before the work is done. The plan is to use the data collected from the USBF to create a “cheat sheet” to determine how much a repair can be expected to add to the weight of the blade.
“That way, when I evaluate a blade, I can see that blade has XYZ weights on it, and when the aircraft examiner recommends a repair, we have a playbook that says this repair weighs this much,” Peedin said. “You can see how the blade is currently weighted and get an educated guess as to whether we can still balance the blade when this is done.”
Engineers say this preplanning will help eliminate the time and money spent on unsuccessful repairs.
“There’s always a concern with blades that need a lot of repairs that we will put in the effort for a repair only to find we can’t balance the blade due to the additional repair weight,” said Barnard. “This will allow us to know whether a blade can ultimately be balanced before we perform the repairs.”
Peedin said adopting the static balance fixture system for the CH-53K blades has taken some time to validate, because the whirl towers have such a long history of balancing blades. However, he said the feedback the USBF system is getting makes his efforts in advocating for the system worthwhile.
“My biggest satisfaction was when we got the first two units here, and we started putting blades on them and getting consistent data,” Peedin said. “A great deal of effort has gone into procuring this system, and it’s gratifying to see evidence that we’re going in the right direction.”
FRCE is North Carolina’s largest maintenance, repair, overhaul and technical services provider, with more than 4,000 civilian, military and contract workers. Its annual revenue exceeds $1 billion. The depot provides service to the fleet while functioning as an integral part of the greater U.S. Navy; Naval Air Systems Command; and Commander, Fleet Readiness Centers.
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